1. Field of the Invention
The present invention relates to a controller of a stepping motor and its control method and, in particular, to a power-saving type controller which is useful for driving a stepping motor in an electronic timepiece, and a control method of the stepping motor.
2. Description of the Related Art
Stepping motors, also called pulse motors or digital motors, are driven by a pulse signal and find widespread use as an actuator in digital control devices. Compact electronic devices and information handling apparatuses, appropriate for portable use, have been recently developed, and miniature and light-weight stepping motors are widely employed in these devices. Timing devices such as an electronic timepiece or watch, and timing switches are typical of such electronic devices. In the timing device, an oscillator circuit employing a crystal oscillator generates a reference pulse, which is then converted into a timing signal having a frequency appropriate for timing purposes, for example 1 Hz. A drive pulse is supplied to the stepping motor in synchronization with the timing signal to drive a second hand in the timing device.
Since a power supply for use in such a portable electronic device is subject to limited space allocation and other limitations, it is important to reduce power consumed by a stepping motor and the like for reliable operation and extended life of the device. For this reason, in an electronic timepiece employing a stepping motor, the root-mean-square value of a drive pulse supplied to the stepping motor is automatically set to an appropriate value to match a condition unique to each electronic timepiece or operational conditions. The power consumption by the stepping motor is thus reduced. Several methods for controlling the root-mean-square value of the drive pulse are available. In one method, the drive pulse is controlled by changing its pulse width or pulse height. In another method, the drive pulse is formed of a plurality of sub-pulses, and the root-mean-square value of the drive pulse is controlled by changing the duty factor of the sub-pulses.
In addition to the reduction in the power consumption by the stepping motor, every attempt is made to reduce the overall power consumption by the electronic device. It is also contemplated today that the power consumed in an oscillator circuit is reduced by lowering the oscillation frequency of a reference pulse (reference signal) output by the oscillator circuit employing a crystal oscillator, for example. By lowering the frequency of the reference pulse, the component count of a frequency divider, for example, is reduced, and the reduction in the operational frequency in the circuit in turn reduces the power consumption.
A controller that controls the root-mean-square value of the drive pulse according to the duty factor of the sub-pulses suffers a drop in the control resolution of the duty factor of the sub-pulses when the frequency of the reference pulse fed by the oscillator circuit is lowered. Specifically, when the frequency of the reference pulse is 32 kHz, the duty factor of a pulse signal of 1 kHz can be controlled in step sizes of 1/32 (resolution). When the frequency of the reference pulse drops to half, namely, to 16 kHz, the duty factor is controlled in step sizes of 1/16, and the control resolution is substantially degraded. This makes it difficult to control the root-mean-square value of the drive pulse to a small but sufficient current matching the operational state of the stepping motor. To prevent a timepiece hand from running in an erratic fashion under insufficient power of the drive pulse, the electronic timepiece is supplied with the drive pulse of a high root-mean-square value rather than narrow step-sized drive pulse based on a high-frequency reference pulse. This increases the power consumption by the motor, and the timing device therefore fails to take advantage of the power savings available with a low-frequency reference pulse.